Recently, along with high performance, high function and network development of information and communication apparatus, operation signals tend to high frequency for high-speed transmission and processing of large capacity of information. The rise of the use frequency of electronic products requests a lower dielectric constant of the baseboard material, a less dielectric loss and a better homogeneity of the dielectric constant of the baseboard.
Resins with a low dielectric constant are used in the high-frequency circuit baseboard at present to obtain a better high-frequency performance. These resins include polyphenylether, cyanate, thermosetting resins consisting of elemental carbon and hydrogen and comprising carbon-carbon unsaturated double bonds, PTFE and the like. As for the copper-clad plate, glass-fiber cloth is generally used as the reinforcing material. However, the lowest dielectric constant of glass-fiber cloth is only 3.7 (Q-glass). Due to a great dielectric constant of glass-fiber cloth, it is difficult to lower the dielectric constant of the copper-clad plate made from other resins than PTFE.
On the other side, woven materials are used as the reinforcing materials in the current high-frequency circuit baseboard (e.g. glass fiber cloth). Due to weaving and the presence of nodes of the crossing parts of the woven fibers, the dielectric constant in the circuit board is not isotropic in the planar X and Y directions, and there is the dielectric constant difference in the X and Y directions. During the transmission of such high-frequency signals in the high-frequency circuit baseboard, different dielectric constants in the X and Y directions will result in signal attenuation so as to affect the signal transmission stability.
U.S. Pat. No. 6,218,015 discloses combining two polytetrafluoroethylene resins, mixing, filling and casting into films to produce a circuit baseboard. Due to the use of thermoplastic polytetrafluoroethylene resin, the circuit material produced by such method has excellent dielectric properties, and there is no difference in the dielectric constants in the X and Y directions. However, cracks readily occur and the rate of finished products is not high when such casting method is used to produce thicker films. Especially when circuit boards having a greater thickness are required, many layers of films need to be laminated, thereby resulting in a low production efficiency.
U.S. Pat. No. 4,772,509 discloses impregnating polyimides with a porous, expanded polytetrafluoroethylene film to produce a prepreg, then to produce a circuit baseboard. U.S. Pat. No. 5,652,055 discloses impregnating a thermosetting resin with a porous, expanded polytetrafluoroethylene film to produce a circuit baseboard. However, since these two patents disclose producing a circuit baseboard with the thermosetting resins having a great dielectric loss angle tangent (which is greater than 0.01), their dielectric properties are worse than the circuit baseboards produced by using polytetrafluoroethylene resin in U.S. Pat. No. 6,218,015.